1. Field of the Invention
The invention relates to a magnetic resonance apparatus which includes a coil system for generating a steady magnetic field in a measuring space of the apparatus, a gradient coil system which is arranged within the coil system for generating the steady magnetic field and serves to generate a magnetic gradient field in the measuring space of the apparatus, which gradient coil system includes an inner coil and an enclosing shielding coil for shielding the environment from the gradient field generated by the inner coil, which gradient coil system has been optimized so as to provide internal compensation of the Lorentz forces exerted on the turns of the gradient coil system by the steady magnetic field.
2. Description of Related Art
An apparatus of this kind is known from German patent No. 195 27 020 C1.
In order to optimize the shielding effect of the shielding coil of the gradient coil system, it is customary to proportion the shielding coil in a magnetic resonance apparatus in such a manner that the shielding effect of the shielding coil is maximum, given the limitations imposed by the apparatus design. However, as a consequence of the choice for this optimization, such a coil system is not optimized for the compensation of Lorentz forces acting on the coils, so that upon excitation such coils produce noise of a level which is usually annoying to the users.
The cited patent mentions that the ratio of the number of turns of the inner coil to that of the shielding coil of the gradient coil system can be chosen in such a manner that the gradient coil is optimized in respect of Lorentz forces. However, the cited document does not disclose any further steps for implementing this idea.
The effect of optimization of the gradient coil in respect of Lorentz forces is that the forces acting on the outer coil (the shielding coil) are equal and opposed to the forces acting on the inner coil. When these two coils are connected to a supporting member so as to form a rigid unit, as is customary, these forces compensate one another and, consequently, the mechanical vibrations produced by the gradient coil system will be considerably reduced, resulting in a substantial reduction of the annoying noise.